The Physics of Vision
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Image and Color

An Upside Down Image


Refraction in the eye causes light rays from a point on an object to converge to a focal point on the retina. Many light rays from many different points on an object converge on the retina forming an image of the object.

The image formed on the retina is upside down (shown in the picture on the right). This is due to the direction of the light rays as they enter the eye, and the nature of the converging lens which do not bend light rays as they enter the center of the lens.

The retina functions as the light detector of the eye.

Rods and Cones

The retina has two types of nerve cells that can sense light:
  • Rods- located at the outer edge of the retina. There about 120 million of these cells in the eye and they are the most sensitive to light, which make them good motion detectors and for vision in dim lighting. However they are not sensitive to different colors.
  • Cones- concentrated near the center of the retina. There are 6 to 7 million of these cells in the eye and they are responsible for detecting color. The fovea centralis, located at the very center of the retina, contains closely packed cones and no rods, which provide the best resolution and sensitivity to color.

Rods and Cones cells on the retina contain photosensitive chemicals. The rods contain the chemical rhodopsin, and the cone contains cone pigments. There are three types of these pigment: red, blue, and green. Each cone has one of these pigments and is therefore sensitive to the corresponding color.

Visible Light Spectrum

It is important to note that color is a perception, not a physical entity. The appearance of color is due to the wavelength of light. Therefore each cone is sensitive to different wavelengths of light, and this is then interpreted by the brain as color. The image to the left shows the wavelengths of light and their corresponding color.


Once the light rays produce an image on the retina, chemical reactions occur at the rods and cones which then transmits electrical impulses to the optic nerve. The image is then sent to part of the brain called the primal visual cortex which is responsible for interpreting the image. This is where the image is flipped right side up.